This invention relates to binders preferably cured at room temperature which are formed by mixing (a) a binding composition or material comprising ethylenically unsaturated monomers, ethylenically unsaturated polymers and blends of such unsaturated monomers, ethylenically unsaturated polymers and blends of such unsaturated monomers and polymers, and (b) a free radical initiator comprising a peroxide and a catalytic agent. In general, this invention relates to room temperature curable binding compositions which are polymerizable through free radical initiation and chain extension. These binding compositions are useful in adhering materials and especially particulate solids. In particular, the invention relates to compositions which are capable of bonding sand or other aggregates to form molds or cores for casting metals, including especially aluminum and other lightweight metals. Molds and cores made using these binders demonstrate superior collapsibility when used in casting lightweight metals, i.e., metals which are cast at low casting temperatures. The curing of the binding material to form the binder composition preferably takes place at ambient temperature and is accomplished by a free radical initiator comprising a peroxide and a catalytic agent. In preferred form, the catalytic agent is gaseous and the cure or hardening is nearly instantaneous. However, selection of differing catalytic agents results in a variety of options for the manner and rate of cure.